1. Field of the Invention
This invention relates to novel acid-functional polymers obtained by reacting a hydroxy-functional cellulose ester copolymer with a cyclic dicarboxylic anhydride to produce acid groups and ester groups. The hydroxy-functional cellulose ester copolymer comprises the graft copolymer reaction product obtained by polymerizing an ethylenically unsaturated alcohol, and optionally, at least one other copolymerizable ethylenically unsaturated monomer in the presence of a cellulose ester, e.g. cellulose acetate butyrate (CAB) polymer.
This invention also relates to novel reactive coatings which utilize the acid-functional polymers. The coatings can be cured at room temperature or force dried at temperatures ranging up to about 350.degree. F. The coatings may be utilized as primers, topcoats or as clearcoats and/or basecoats in clearcoat/basecoat compositions. The coatings involve the combination of the carboxylic acid-functional cellulose ester based polymers with an epoxy-functional compound. The coatings may, optionally, also include an anhydride-functional compound and, optionally, also hydroxy-functional compound. These combinations provide fast reacting, durable coatings which minimize the toxicity problems which may be associated with other low temperature curing systems.
2. Description of the Prior Art
Graft copolymers obtained by the free radical addition polymerization of cellulosic materials and ethylenically unsaturated monomers to produce hydroxy-functional polymers has been known in the art. U.S. Pat. No. 3,883,453 teaches coating compositions containing a copolymer of mixed alkyl esters of acrylic or methacrylic acid polymerized in the presence of cellulose acetate butyrate and teaches that this hydroxy-functional polymer can subsequently be cross-linked with an amino resin. U.S. Pat. No. 4,358,510 teaches metallic paint films utilizing as a vehicle component a modified vinyl-type graft copolymer obtained by graft copolymerizing cellulose acetate butyrate with one or more vinyl-type unsaturated monomers. Although these prior art hydroxy-functional vehicles have utility when combined with curing agents reactive with the hydroxyl groups, they do not teach low temperature curing reactive systems having minimal toxicity. The acid-functional vehicles of this invention provide unique reactivity, solvency and performance improvements over the hydroxy-functional cellulose ester based graft copolymer vehicles of the prior art.
One common prior art approach to high performance low temperature curing coatings has involved two-component coatings comprising reactive isocyanates and active hydrogen-containing compounds such as hydroxyl-containing polymers or amine-containing polymers to produce polyurethane or polyurea coatings. Although these materials have excellent performance and cure at low temperatures, the isocyanates may, under some conditions, be relatively hazardous to handle.
Coating compositions comprising reactive combinations of epoxy-containing compounds and compounds having acid or amine functionality are known in the art. Similarly, coating compositions comprising cyclic anhydrides and hydroxy-functional compounds are also known in the art. The prior art has not, however, taught the novel acid-functional polymers of this invention nor has it taught coating compositions comprising these acid-functional polymers with epoxy-functional compounds and, optionally, anhydride-functional compounds, and, optionally, hydroxy-functional compounds to provide low temperature curing coatings having excellent durability and performance.